There has been so much conversation about hps vs LED.. while at work today and thinking about my next build, i just ended up going back to basics and it just makes a little more sense now.
1000w HPS = 110 000 lumens = 110lm/w
For a LED to be exactly 2 for 1 you would need 220lm/w to get the same lumens output.
If you run your LED at 170lm/w then you do 110k / 170 = 647W
This seems very simple but yet I think its one of the best math you can to compare the two. It doesnt account for all factors (spectrum, some will say "penetration", size, spread, etc).. but the numbers are there. LED will provide better uniformity because you can spread them so much more over your canopy and the hps is 360 degrees so you loose more off the reflection but all in all simple math for the win!
The problem with that is lumens dont grow plants. We want to look at umol/j using PAR. Lumens = light for humans, PAR = light for plants. PAR stands for photosynthetically active radiation.
HPS is on avg. 1.3 umol/j
Todays LEDs are on avg. 2.3umol/j
So if you had a 1000w HPS, you would have 1300PPF of output (1000 * 1.3). PPF is Photosynthetic photon flux, aka how many raw photons in the PAR range that are emitted from the light.
And lets use 500w of LED strips, you would have 1150 PPF of output (500 * 2.3).
Now, this is a curveball I know, but PAR doesnt tell the whole story. The spectrum of the light must be taken into consideration when comparing PAR between 2 different lights. If those LEDs were all 6000k, the PPF would be the same but obviously 6000k wont yield as much as the 2100k HPS at almost the same raw output.
But we usually use 3000-3500k for LEDs, which packs in considerably more red, far red, and green wavelengths in the spectra than the 2100k HPS does (and also blue to keep plants growing naturally and not stretched). HPS is mostly yellow, with red and far red being the rest of the main spectra.
So even if the PPF output is identical, the LEDs contain more of the wavelengths that drive the most photosynthesis. So theory says the 3000k LEDs should perform better at the same PPF as the HPS. HPS does contain heavy levels of IR, which mix with the far red to make the emerson effect which is important too. But you can add IR to the LEDs of course.
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So hopefully that explains how people are achieving the results of an HPS, while only using half the watts. The raw numbers tell us the 1000w HPS should beat the 500w of LED strips since the PPF is higher. But the spectra of the LEDs drive more photosynthesis than the HPS spectra, so it can close that raw PPF output gap.
And that's not to mention the fact LEDs have been proven over and over (and over and over...) to increase cannabinoid and terpene content across the board over HPS. Which for extractors who make hash, is a HUUUUUGE deal.